It has been long known that a fatty acid alkyl ester is obtained by transesterification of an alkyl alcohol with a mono-glyceride, di-glyceride and tri-glyceride (these are generically called fatty acid glyceride) which are main components of vegetable oils, animal fats and used fats and oils of them (for example, “Organic Chemistry Handbook,” Gihodo Publication, 1988, pp. 1407 to 1409). Furthermore, various investigations have been made on a technology of producing a fatty acid alkyl ester which can be used as a diesel fuel from fats and oils, utilizing this reaction.
As a method for industrially producing a fatty ester from a fatty acid glyceride, there are long known methods in which a fatty acid glyceride is once hydrolyzed to be converted into a fatty acid, then, in the presence of an acid catalyst or enzyme catalyst, the fatty acid is further subjected to a dehydration reaction (esterification reaction) with alcohols under an anhydrous condition to be converted into a fatty acid alkyl ester, however, these are scarcely used as an industrial production method due to low reaction rate. Currently, in industrially often used methods, a fatty tri-glyceride is subjected to a transesterification under an anhydrous condition in the presence of an alkali metal catalyst under normal pressure at ambient temperature or temperatures near the boiling point of an alcohol with shorter alkyl chain. However, since, in this reaction, an alkali metal catalyst is dissolved in the reaction solution, there is a problem that the alkali metal catalyst is dissolved in a solution of the product and separation and recovery thereof are difficult.
Further, waste oils and the like often contain water, consequently, removal of water in a raw material is inevitable as a pre-treatment, in use of the above-mentioned alkali metal catalyst method. It is general that natural fats and oils contain a free fatty acid, and the content of a free fatty acid differs depending on the origin of a raw material and its treatment method. For example, a waste edible oil contains 3% or more of fatty acids and palm oil from an oil pressing process contains 5% or more of fatty acids. When an alkali metal catalyst is used under a condition of inclusion of a large amount of free fatty acids, an alkali soap becomes a by-product and an alkali metal catalyst in an excess amount is necessary, alternatively, there occurs a problem that separation of a fatty acid ester layer and a glycerin layer is difficult due to a by-product alkali soap, and the like. Because of these reasons, when a transesterification of a fatty acid glyceride is conducted in the presence of an alkali metal catalyst, a pre-treatment process is necessary for removing a free fatty acid.
From the standpoint of avoiding such a problem, for example, Japanese Published Unexamined Patent Application No. S61-14044 discloses also a method of converting into an ester of a free fatty acid with the aid of an acid catalyst, as a pre-treatment process. In this method, a free fatty acid is converted into an ester as a pre-treatment for conducting a transesterification of a fatty acid glyceride in the presence of an alkali metal catalyst, however, there is a problem that removal of an acid catalyst is necessary before performing the next transesterification of a fatty acid glyceride and when an acid catalyst remains, it is neutralized, consequently, the use amount of an alkali metal catalyst increases corresponding to the neutralization amount.
As a method for producing a fatty ester not requiring the above-mentioned pre-treatment process, there are also suggested methods using a solid acid catalyst (for example, Japanese Published Unexamined Patent Application No. H6-313188). However, an acid catalyst has a critical defect that the reactivity thereof for a transesterification of fats and oils is lower as compared with that of an alkali metal catalyst, and there is a problem that a large amount of catalyst is necessary in a transesterification using an acid catalyst.
On the other hand, there are recently also suggested so-called supercritical methanol methods in which a transesterification of fats and oils is conducted under supercritical conditions for alcohol without using a catalyst (for example, Japanese Published Unexamined Patent Application No. 2000-204392, 2000-109883). However, in the supercritical methanol method, there is a disadvantage that a large excess amount of alcohol should be present and higher temperatures of 300° C. or more are necessary, for efficient progress of a transesterification. Further, in this method, an effect of the presence of water in the reaction system has not been confirmed.
The present invention has been made notifying such conditions, and an object thereof is to solve a problem of separation and recovery of catalysts present in an alkali metal catalytic method currently often used, a problem of excess consumption of a catalyst by a free fatty acid in a raw material, and a problem of decrease in transesterification due to water in a raw material and to solve a problem of the presence of a large excess amount of alcohol in a conventional supercritical methanol method, and the present invention provides a method for producing a fatty acid alkyl ester, that is effective for conversion into a fatty acid alkyl ester from raw material oils which cannot be treated by conventional technologies such as a dark oil containing a free fatty acid as a main component discharged particularly from a purification process in an oil production factory, and a waste edible oil having a high free fatty acid content and/or water content, and the like.